The invention relates to a method and an apparatus for suppressing optical beat interference noise, in particular to a method to suppress pump mode optical beat interference in Raman amplified WDM systems.
When light is transmitted through an optical fiber the light signal can be scattered by the vibration of the glass structure of the optical fiber. A Raman amplifier is a type of an optical amplifier used in fiber optic transmission systems. The Raman amplification is based on the stimulated Raman scattering phenomenon where a lower frequency signal photon uses a scattering of a higher frequency power pump photon in an optical medium in the non-linear regime. As a result of this, another signal photon is produced, wherein the surplus energy resonantly passes through the vibrational states of the medium. Raman amplifiers can be deployed in ultra long-haul, long-haul, regional and metro-core fiber optic transmission systems. Distributed Raman amplifiers improve the optical signal to noise ratio and reduce the non-linear penalty of fiber systems, allowing for longer reach, longer amplifiers spans, higher bit rates, higher number of channels, closer channel spacing and operation near the fiber zero dispersion wavelength. Raman scattering of internal light with photons of the gain medium produces photons coherent with the incoming photons. In a Raman amplifier the optical signal can be amplified by Raman amplification.
Different types of Raman amplifiers are known, e.g. a distributed and a lumped Raman amplifier.
A distributed Raman amplifier is a Raman amplifier in which the transmission line optical fiber is utilized as the gain medium by multiplexing a pump wavelength with a signal wavelength, whereas a lump Raman amplifier utilizes a dedicated self-contained shorter length of optical fiber to provide amplification. In the case of a lumped Raman amplifier a highly non-linear fiber with a small core such as a dispersion compensating fiber is utilized to increase the interaction between the signal and the pump wavelength. The pump light may be coupled into the transmission fiber in the same direction as a signal (so-called co-directional pumping), in the opposite direction (counter-directional pumping) or in both directions (bidirectional pumping).
In a co-pumped Raman pump signal relative intensity noise transfer limits the system signal to noise performance of wavelength division multiplexed (WDM) signal channels.
When using polarization multiplexed pump lasers of similar wavelengths optical beat interference, OBI, can induce sudden and sporadic signal gain and noise variations on the WDM channels. The pump optical beat interference coupled with a non-linear pump signal transfer process within this optical fiber is responsible for this phenomenon. Distributed Raman amplifiers are sensitive to the sudden and large noise power variations which can cause receivers to loose clock and data recovery for a long period. Large noise power variations may occur randomly over time or at some Raman amplifier gain settings. Accordingly, there is a need to provide a method and apparatus for suppressing optical beat interference noise in an optical network using Raman amplifiers having polarization multiplexed pump lasers.